Method for forming a wear-resistant surface on a metal article

ABSTRACT

A METHOD IIS DISLOSED BY WHICH A METAL WORKPIECE IS COATED WITH A WATER-RESISTANT LAYER WHICH IS METALLICALLY BONDED TO THE SURFACE OF THE BASE METAL AND WHICH IS SUPERIOR IN ABRASION-RESISTANT ABILITY TO THE COATINGS WHICH ARE FORMED BY PRIOR ART METHODS, WHEREIN THE METHOD COMPRISES APPLYING A COATING OF A SELF-FLUXING ALLOY ON THE SURFACE OF THE BASE METAL AND SUBJECTING THE COATING TO NITRIDING TREATMENT OR TO NITRIDING AND CARBURIZING TREATMENT.

m c n m m B M A m Mm mm IA HM n a mm m W AA m I m P a F D m m B Aug. 6, 1 $4 File'd Aug. 8, 1972 3 Sheets-Sheet 1 6E hT ROOM Z5 Fig.

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i 7 YASUNORI SHIMODA E A!- METROD 1 0R FORMING A WEAR-RESISTANT SURFACE ON A BETA]; ARTICLE Filed Aug. 8, 1972 3 Sheets-Shoat 2 YASUNORI SHIMODA ETAL umaon FOR FORMING A. wmn-nasrsnnr sunmcz on A warm. mum: Filed Aug. 8, 1972 3 Sheets-$hewt 3 C i i 7".. r ME HOD F R A WEAR-RESISTANT ME A Yasunorifihimoda Tayhikawa, and Kojlro Taniguchi, I' ma, Japan, assignors'to Nissan Motor Company, Limited,'Yokohanfm' lapan" FiledA'ng; 8;"1972','S" .0. 278,758

an, Aug. 9, 1971,

7/00y'9fi00, 11/14 a i rity; tippli6catiorij m Claims *ansmor or DISCLOSURE ,A method is disclose hy which a metal workpiece is coatediwith a wearr resistant, layer which is metallically bonded t'o surface of the base. metal and which is superior in abrasion -resistant abilityto the coatings which arenformedhbyprior .ant methods,-wherein the method comprises applying a co,ating ofa self-fluxing alloy on the surface of the base metal and subjecting the coating to nitriding treatment onto nitriding and carburizing treatment.

The-present 'invention relates'to surface treatment of metalobjects and, moreparticular'ly, to a method for formn'g ';a"wear-'resistant coating on a metal object.

which-machine and autcir'noti'v' parts to be subjected to abrasive wear"caus'edby g'ailingand seizing are coated by fiame"- spraying* processes-with layers of wear-resistant properties= which are' congruous with different operating ranges and to achieve different surface effects. Materials by fusion. The presence'ldfz'the pores in the coating results in reduction of the corrosion resistance of the workpieces while' 'theWriclranica "bond hetween the coating and' thej lbase mtalteniis to be a"cause for the coating being peeieden the hase'metal;

These drawbacks are e'liminatedwherethe self-fiuxing alloys are used as thelmaterial for the abrasion-resistant suchas"for'e'kample an alloy of nickel, chromium, boron and silicon are spr ayed onto surface of a metal work piece an'djthe the workpiece is subjec'ted toiiielting treatmentfo'r' r g, the pa rticles of the alloy. Porosity imthe cgating isoverfco ne by this melting treatment a'nd,,..fur A rrnore the c'oating is bonded; by fusio the surface f. tlrebase metal, ,this instance a iis f na l tw enth ,aii a ts ns w m n eme ing and the oxides present in the base metal and floats O !1,; 65

the-surface gof a the tetra ting? FEMS it auses the coating to; be tig to e -1 mate. the voids which gareeusuallysiformed tin the-vcoatsiformed hyi the spraying method;

with use '0 the *self Tllm'tmetahobjecte thusaprepared A witle'wariety of 'methods 'arepresently in use by and,' moreover,'i the coatingis. bonded to the surface of the base metal merely in a mechanical fashion, viz, not

gfused; netaonl ato th basemetal. bnttinitselfisor asm i0 fluxirig1alloysrhave usefulapractidal'applications inmu'meri on quarters ofthe'industry wherethefequiretnentfortlie 3,827,920 Patented Aug. 6, 1974 abrasion-resistant and corrosion-proof properties are not very exacting. In the motor vehicle production industry, for instance, such materials have been successfully used for forming parts which are to be subjected to sliding and frictional motions. There is, however, an ever increasing demand for materials having still improved abrasion and corrosion resistances in the motor vehicle industry so as to meet the requirement for the performance quality of the motor vehicles which is becoming more and more many-sided. It is quite likely that such trend will continue in the future and, as such, the conventional surface treatment using the self-fluxing alloy is becoming obsolete.

It is, therefore, an important object of the present invention to provide an improved method for forming a wear-resistant coating on a metal object to meet the exacting anti-abrasion requirements.

It is another important object of the invention to provide a method by which a metal object is coated with a wear-resistant layer which is bonded by fusion to the surface of a metal object and which is superior in an abrasion-resistant ability to the coatings thus far formed by ing or to both nitriding and carburizing. The base metal h operable by this method may be steel, cast iron, cast steel, copper, any one of various copper-based alloys, or a certain type of nickel alloy such as Monel metal. The selffluxing alloy, on the other hand, may be based on nickelchromium (Ni-Cr), cobalt (Co), tungsten monocarbide (WC), or cobalt-iron-chromium (CoFe-Cr). The coating of the self-fluxing alloy is usually applied to the surface of the base metal by flame spraying of the alloy and the resultant coating is then'heated and melted by the use of an oxygas or oxyacetylene torch, by lowor highfrequency induction heating, or in an electric heating furnace. Or otherwise, a layer of the self-fluxing alloy may be further applied to the coating by means of weldspraying.

The coating thus formed on the surface of the base metal is then subjected to nitriding treatment or to both nitriding and carburizing treatments in an atmospheric furnace or a liquid heat-treatment furnace. Where it is desired to have the coating processed in the liquid heattreatment furnace or a salt bath; it is preferable that the 1 workpiece is immersed in a liquid medium such as potassium cyanate '(KOCN) which is heated to a temperapurpose of increasing the resistance to abrasion of an object of iron or an iron-base alloy. The improvement of the abrasion resistance achieved by this process is considered to-result from a mechanism which is described" below.

' When the workpiece of iron or an iron-base alloy=is processed in. the salt bath, the liquid medium in-the bath is decomposed to nitrogen and carbon. The nitrogen and carbon thus produced diffuseinternally of thewofkpiece from the surface thereof? wl1'ereuporr' an upper layer -of- 20 microns "is "formed" at tlie' s'urface of the workpiece"; This' laye'r contains "therein nitrogen a'rid an appreciable Y amount o'f carbori in the forms-of: trif'er ro =moiionitride 'bide '(;Fe ,C)'. The nitrogen vvhi'ch i presen'tfunderneath "supersaturated and; dissolved in a:

' nstituent'and consequently dea whitish compound of a depth of approximately 10' to piece lend themselves to raising the abrasion resistance of the base metal of iron or iron-base alloy.

In contrast to the conventional nitriding or carburizing process in the salt bath in which the base metal per se is processed, the method according to the present invention is characterized in that not the base metal but the coating on the base metal is processed for nitriding or for both nitriding and carburizing. Since, thus, the nitriding or carburizing process proceeds substantially in the absence of iron which plays an important role in the process or in some cases in the presence of a negligible amount of iron which amounts to about 35 percent at a maximum, the mechanism of the process above described will not be applicable to the processing carried out on the coating of the self-fluxing alloy. When, more specifically, the coating of the self-fluxing alloy is subjected to the nitriding process in the salt bath or in an atmospheric furnace in carrying out the method according to the present invention, formation of the layer of such compounds as the triferro monocarbide, tetraferro mononitride and triferro mononitride as produced in the case of the process of nitriding the iron or iron-base alloy does not result.

Experiments were therefore conducted by us in an attempt to determine acceptable compositions of the nitrided self-fluxing alloy coating exhibiting a hardness which is higher than the hardness of a coating of the similar composition but not subjected to the nitriding process. The experiments have revealed that an increase of the order of 200 in terms of the diamondpyramined hardness number is achieved if the nitrided self-fluxing alloy contains at least 8 percent by weight of chromium.

Thus, in the course of nitriding the coating of the selffiuxing alloy as practiced by the method according to the present invention, the chromium in the alloy reacts with the nitrogen or carbon at the surface of the coating so that monochromium mononitride (CrN) and dichromium mononitride (Cr N) are produced. These reaction products precipitated and distributed in close proximity to the surface of the coating of the self-fluxing alloy with the result that not only monochromium monoboride (CrB), dichromium monocarbide (CI-2C), monomolybdenum monoboride (MoB), tricobalt monoboride (C0313) and trinickel monoboride (Ni B) which were present prior to the nitriding process but also monochromium mononitride and dichromium mononitride were formed.

This and other material characteristics of the product obtained by the method as compared with those of the prior art products will be visibly ascertained from the photographs in the accompanying drawings in which the self-fluxing alloy of the coating shown in FIGS. 1 to is based on chromium, iron, and cobalt and in which:

FIG. 1 is an X-ray diifractiometric diagram indicating the existence of the chromium nitride in the self-fiuxing alloy coating formed by the method according to the present invention;

FIG. 2 is an X-ray photograph showing the presence of the chromium dipotassium (CrK in the coating of the self-fiuxing alloy as observed by the use of an X -ray analyzer; U

FIG. 3 is view-similar to FIG. 2 but now shows the presence of the nitrogen dipotassium (NK in the coatinga FIG. 4V is e lectron-microscopic photograph showing the surface of the-self-fluxing alloy coating which is .to

be nitrided; .7

FIG. 5.is,.a .yiew but surface of the selfdluxing alloy coating which has vbeen sub-.

i c qdntq ih v i di p wstit a salt b a d" vention and (b.). sh ows;the, rocker arm of .the wear -1. resistant character which-has thus far been commonly used.

From theillustrations of ==EIGS- 1- 10 3, =i-t--is--apparent-- that the chromium constituent in the coating of the selffluxing alloy is distributed therein in a segregated form and that the distribution of the nitrogen is the higher where chromium is present in a-. higher distribution. Inspection of FIGS. 4 .and -'5,a inoreoive'r', will revear that the self-fiuxing coating subjected to"the' nitridihg process, the salt bath displays'a'n increased effect. If,"thus',[the-" workpiece having thenitrided self fluxingall is lubricated, the lubricant cari bef'fir'nily the surface of the worlgpiec an as.a' consequence will teristics 'of 'the lubrienhance the antifrictionaL char cated surface of the workpiece 'which is lto-rbe subjected to sliding motions. The increased effect"of=-thel.coating-' will, furthermore, significantly improve the initial fit of the workpiece, thereby providing a. f urther, enhanced antifrictional characteristicstheworkpiece.

It will be appreciated' from"the foregoing description that the abrasion resistance of the metal'objectwhiclff is processed by the method according-t6 the present invert tion is increased particularly a't-the =sur'face andin the neighbourhood of the surface of the metal objectflt is therefore, preferable for'the iex ploitation o'f sucli1n creased abrasion resistance that the metal object havirigf the self-fluxing alloy coating be"subjected"t'othe nitrid'in process after the workpiece has been machined to the final shape.

The self-fiuxing alloy applicable to the method according to the present invention shouldbe sele cteci,fronn. those which are not based on iron and should ,poreferab ly -y contain at least 8 perecnt of chromium as..a constituent. The experiments have. revealed-thatlhe upper limino fi the proportion of the chromium constituent should ,1) y, approximately 35 percent and contain.siliconina-certai proportion. Preferred examples of sucha'self-fiuxinga loy are as following: A v

(a) Ni-Cr based, containing 1 of chromium, 4.457, a of silicon and 4.0 of iron. H H f; 5

(b) .Co-Fe-Cr based, containing 27% of chromium.* and 1.8% of silicon. p

(c) Co based, containing 20% of chromium and 4.0% of silicon. ,1 s

(d) WC based, containing 11% of. chromium and 2.5% of silicon. I v Y The features and advantages of themetal objecmproc essed by the method accordingto the present invention, will be understood more clearly from the followingde: scription of the examples of-qthe method and-the. tests-z; conducted on the metal objectsobtained by- -the -.exem.-aplified methods.

EXAMPLES Four identical rocker arms of steel for an internalcom- 1; bustion engine were fabricated and. coatingfbysprayir'ig, fine molten particles'of selfvfluxin'g alloyswof different f compositions. The compositions'of the's'elf-fluxing alloys were as following: 1 Ni-Cr based (specimen 1A,), containing Ni, 17%? i Cr, 3.5% B, 4.4% Si, 0.67 other elements. p

Co-Fe-Cr based (specimen "conta1n1ng" 3,"2%' 30% Fe, 27% Cr, 3.0%13,'1;s% si 7.0% Mo and other elem tits". f V Co based '(spe'ci'm'enC) containirig 40%05; 26% Ni, 20% Cr, 3 .0%;'B,'4=.0%*s 5.0% Moan'dother' elements:

WC 'ba'sed spears-em; containing 35% WC, 46% Ni,-*11-%""Cr-, 2'."-5"7' B, -2 57Si, Fe 0 5% '-C and other' elen'ients. a. i 2. The coating of the self-fluxing alloys 'of -the 'above enumerated compositions was 1' then subjected tom elting treatment atatemperatureof about 1,050" '6; andzthe'reafter to the nitriding process in a bathzcqntainingipotassium .12,

c ammo 4 cyanide (KCN) -.and'theated to; a temperature-rangingfrorn 5201 .0. vto 5103- C.; For; thesalgezzofacornparisom othena;

rockenarms such as the prior art;products .were preparedm which had the coatings of the self-fluxing alloys of the above specified compositions but which were not subjected to the nitriding process.

The rocker arms thus prepared were incorporated in four-cylinder internal combustion engines of the same type and tests were conducted with those engines under actual driving conditions. It is, in this instance, to be noted that the rocker arm of the internal combustion engine is an abutment member for a cam of an intake or exhaust valve and is thus subjected to serious frictional motions during the operation of the engine. The results of these tests are tabulated below, wherein the abbrevations IN and EX refer to the locations of the tested rocker arms which were used for the intake and exhaust valves, respectively, and the numerals l to 4 prefixed thereto indicate the order in operation of the engine cylinders in which the particular valves were incorporated. Thus, lIN refer to the location of the rocker arm which was used for the intake valve for the first engine cylinder while ZEX indicate that the rocker arm was associated with the exhaust valve of the second engine cylinder. The wear ratings of the rocker arms are indexed by meaning excellent," meaning acceptable and meaning unacceptable.

Test I Tests were conducted with an internal combustion engine using the rocker arms of the specimens A and B.

The engine was driven at a relatively low speed with use of a lubricant having a relatively low viscosity. The testing conditions were as following:

Operating hours 48 hours.

Engine revolution speed 1100 r.p.m. Lubricant temperature 55-65 C. Lubricant working pressure 3.5-4.0 kgs./cm.

The results of the tests are indicated in Table I.

TABLE I Specimen A Specimen B Invented Prior art Invented Prior art Location HIX 1IN ZEX 2IN 3EX 3IN 4EX 4IN Wear rating-"U The surface of the specimen A processed by the method according to the present invention and those of the prior art are illustrated in (a) and (b), respectively, of FIG. 6, while the surface of the specimen B obtained in accordance with the invention and those of the prior art are shown in (a) and (b), respectively, of FIG. 7.

Test II The engine using the rocker arms of thespecimens C and D were driven for testing under the same conditions as in Test I. The results of the tests are indicated in Table II.

High-speed endurance tests were conducted with internal combustion engines using the rocker arms of the specimens A and B under the following conditions:

Operating hours 100 hours. Engine revolution speed 5000 rpm. Lubricant temperature 83-93 C. Lubricant working pressure 3.5-4.5 kgs./cm.

The results of the these tests are tabulated in Table III.

TABLE III Specimen A Specimen B Invented Prior art Invented Prior art Location IEX IIN 2EX 2IN 3EX SIN 4EX 4IN Wearmtins The surfaces of the rocker arms of the specimen B processed by the method according to the present invention and of the prior art are illustrated in (a) and ('b) of FIG. 8.

The same tests were further conducted with the specimens which were subjected to the nitriding process in an atmospheric furnace, achieving substantially similar re sults. The nitriding proces was carried out under the following conditions:

Temperature 480-650 C. Process hours 5-100 hours. Type of atmosphere Ammonium gas.

It was ascertained by these tests that heating the test pieces to a temperature below 480 C. results in insufficient nitriding of them while satisfactory surface hardness cannot be obtained if the test pieces are heated to a tem perature above 650 C.

What is claimed is:

1. A method for preparing a base metal article selected from the group consisting of steel, cast steel, cast iron, copper, copper alloy and nickel alloy having a wearresistant surface, comprising the steps of spraying onto a surface of said metal article a coating of a self-fluxing alloy comprising a base alloy selected from the group consisting of a nickel-chrome based alloy, at cobalt-ironchrome based alloy, a cobalt based alloy and a tungsten monocarbide based alloy; said self-fluxing alloy containing about 8% to 35% by weight of chromium and at least one self-fluxing component selected from the group consisting of boron and silicon, melting said coating for eliminating porosity of said coating and for tightly fusing said coating onto said metal article and finally heating said coated metal article to a temperature in the range of 480 to 650 C. in a medium containing nitrogen for nitriding said coating.

2. A method as claimed in Claim 1, further comprising the application of another coating of said self-fluxing alloy to the initially formed coating by means of weld-spraying and nitriding the second coating together with said initially formed coating.

3. A method as claimed in Claim 1, further comprising carburizing said nitrided coating.

4. A method as claimed in Claim 1, according to which said coating is nitrided by heating to a temperature ranging from 500 C. to 650 C. in a liquid heat treatment furnace.

5. A method as claimed in Claim 1, according to which said coating is nitrided by heating to a temperature ranging from 480 C. to 650 C. in a gas atmosphere furnace.

References Cited UNITED STATES PATENTS 2,936,229 5/1960 Shepard 75-171 3,677,832 7/1972 Van Thyne et al. 148-20.3 3,180,564 4/1965 Fuhrmann et al. 418-178 3,245,387 4/ 1966 Froede 418--179 3,428,442 2/ 1969 Yurasko 117-105.2

FOREIGN PATENTS 1,811,916 6/1970 Germany 418-113 867,455 5/1961 Great Britain 117105.2 907,355 10/ 1962 Great Britain 117---46 FS CHARLES N. LOVELL, Primary Examiner U.S. Cl. X.R. 

